jamasi

Thanks to your help I got it working.

In case sb. else is looking for the correct hook:

function myExlogHashAuthenticator($password, $hashFromDatabase, $username, $externalUserData) {
  // $salt = exlog_get_option("external_login_option_db_salt");
  $salt = $externalUserData[$db_data["salt"]];

  $algorithm = 'sha1';
  $calc1 = hash($algorithm, $password);
  $calc2 = hash($algorithm, $salt . $calc1);
    
  return $hashFromDatabase = $calc2;
}
add_filter('exlog_hook_filter_authenticate_hash', 'myExlogHashAuthenticator', 10, 4);

After digging into the source of wordpress, I suppose,

function myExlogHashAuthenticator($password, $hashFromDatabase, $username, $externalUserData) {
	$user_specific_salt = $externalUserData[$db_data["dbstructure_salt"]];
	$algorithm = 'sha1';
	$innerHash = hash($algorithm, $password);
	$hashCalc = hash($algorithm, $user_specific_salt . $innerHash);

    return password_verify($hashCalc, $hashFromDatabase);
}
add_filter('exlog_hook_filter_authenticate_hash', 'myExlogHashAuthenticator', 10, 4);

should work. Still I wonder what settings to use in the configuration page to activate it.